1. Field of the Invention
This invention relates to a record medium, an optical disk unit using the record medium, and a recording method.
2. Description of the Related Art
In recent years, various methods of performing multilayer recording on a record medium have been proposed to increase the record capacity.
That is, multiple micro-hologram layers are provided in a plate body in the thickness direction of the plate body; since multiple micro-hologram layers are provided, the record capacity becomes extremely large.
(Refer to Patent Document 1.)
The feature of the conventional example shown in Patent Document 1 is as follows: Since the record medium is provided with multiple micro-hologram layers, an optical disk unit using the record medium for recording and playing back can perform digital recording by applying light to a micro-hologram of a record portion of the micro-hologram layer from one side of the record medium and causing optical alteration to form a portion where the micro-hologram is erased and applying no light to form a portion where the micro-hologram is left.
At the playback time, light can be applied from one side of the record medium to the portion of the micro-hologram layer where the micro-hologram is erased and the portion where the micro-hologram is left and reflected light therefrom can be read for performing digital playback.
In contrast, in examples preceding the conventional example, to form a micro-hologram on a record medium, light must be applied from both sides of the record medium for causing optical interference to occur to form a micro-hologram. At the playback time, in the examples preceding the conventional example, light can be applied from one side of the record medium to the micro-hologram of the micro-hologram layer and reflected light therefrom can be read for performing digital playback.
That is, according to the conventional example shown in Patent Document 1, the optical disk unit for recording and playing back has the feature of a simplified structure because a light supply path needs only to be provided on one side of the record medium.    Patent Document 1: U.S. Pat. No. 7,388,695
As described above, according to the conventional example shown in Patent Document 1, a light supply path needs only to be provided on one side of the record medium for the optical disk unit for recording and playing back the record medium, so that the structure is simplified.
However, such a conventional example involves a problem of difficulty in performing tracking control.
That is, in a record medium having multiple micro-hologram layers, particularly a micro-hologram layer of an inner layer cannot be given tracking data and thus tracking data is provided on the surface of the record medium.
However, this kind of record medium is provided with an extremely large number of internal micro-hologram layers as described above and if tracking data is provided only on the surface of the record medium in spite of the state, it may become difficult to perform tracking control at the time of recording and playing back a micro-hologram layer of an inner layer.